Animate quantity steppers and make the nav bar sticky

- Add an AnimatedNumber component that slides a quantity stepper's
  count horizontally — left on increment, right on decrement — and
  wire it into both the menu and basket steppers.
- Make the top nav bar position: sticky, with an explicit background
  so scrolled content doesn't show through it.
- Reserve the scrollbar's gutter on every page (scrollable or not) so
  the sticky header doesn't shift width when navigating between short
  and tall pages.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/components/ordering/MenuList.tsx

@@ -1,3 +1,4 @@
+import { AnimatedNumber } from '#/components/ui/AnimatedNumber.tsx'
 import type { MenuItemWithRecipe } from '#/domain/ordering.ts'
 
 import styles from './MenuList.module.scss'
@@ -51,7 +52,7 @@ export const MenuList = ({
             >
               −
             </button>
-            <span className={styles.count}>{quantity}</span>
+            <AnimatedNumber value={quantity} className={styles.count} />
             <button
               type="button"
               className={styles.stepperButton}